The so-called near-far problem is well documented in certain wireless communications systems, such as cellular telephone systems. This problem is an artifact of a point-to-multipoint architecture wherein base stations are located in cells. In this architecture, each base station services a number of subscriber units, such as portable handsets, located throughout a cell. The remote subscriber units may be located at various distances from the base station within the cell, and may even be mobile within a cell or between cells.
It becomes desirable in such systems, and in particular, those making use of Code Division Multiple Access (CDMA) type modulation, for precise control over the power levels of the transmitted signals. This is in particular important for reverse link signals, that is, the signals traveling from the subscriber units towards the base station. It is desirable for these signals to all arrive at the base station with approximately the same power level, although they may have to travel over various distances. This is advisable in general for a point-to-multipoint system, but particularly important where CDMA codes are used, because the mathematical premise behind the correct functionality of CDMA coded signals assumes that all transmitters are operating at the same power level. The most widely used approach to solving this problem is for the base station to precisely control the transmit power level at each originating subscriber unit. However, even with precise power level control, subscriber units located in outlying areas of the cell must transmit at higher power levels than do the units located closer in to the base station. The so-called near-far problem thus originates from the fact that the high power transmissions from units located at the outlying areas of the edges of the cell may interfere with the operation of the lower power units closer into the base station.
Of course in a cellular system there are typically many cells each of which are adjacent to other cells and which may have overlapping areas of coverage of other cells. In such a system, mobile units located at the far edges of the cell operating at high power may not only interfere with units located in their own cell, but also with other mobile units located in the adjacent cell. Thus an uncoordinated transmission originating from such a mobile unit operating at high power on the edge of the cell may not only degrade the performance of other units within its own cell, but also affect the performance of other mobile units located in adjacent cells as well.
While this can be tolerated to some extent in analog systems, for CDMA systems, this not only results in interfering with individual communications between individual mobile units and the base station but may also affect the available capacity of the entire adjacent cell. In other words, in CDMA systems, interference between the operation of different mobile units causes degradation in the performance of other units to the extent that the available data rates are reduced. That is, the codes used in a code division multiple access system are based upon a mathematical premise that the individual signals arrive at the receiver with approximately the same power levels. If the signals do not arrive with equal power, errors will occur in the decoding process.